1. Field of the Invention
This invention relates to electronic systems and, more particularly, to common-mode detectors.
2. Description of the Related Art
Automatic test equipment (ATE) is used to test electric circuits prior to completing their manufacture. Although differential component detectors are relatively common in ATE, the use of common-mode detectors is relatively new. One such common-mode detector is described by C. Bishop in U.S. Pat. No. 6,281,699. A window comparator generates an output indicative of whether the common-mode signal is above, within, or below predetermined thresholds to determine a common-mode component. Two buffers and a resistor pair are provided with the window comparator to extract the common-mode signal and reduce loading effects on input signals from a device under test (DUT).
Unfortunately, a substantial current can flow through the resistor pair when the comparator's input signals are separated by a large differential voltage. Thus, the buffers should be designed to source and sink the maximum expected current that can flow. Additionally, the resistors must be made low in value in order to operate the circuit with high bandwidth. This adds to the problem of excessive current flow and can also cause significant nonlinearity in the response of the buffers.
There still exists a need, therefore, for a common-mode detector that does not suffer from large currents resulting from large input differential voltages and which does not suffer from high output impedance that precludes high bandwidth operation.